import os
print(os.getcwd()) #返回当前路径
os.chdir("./files") #改变当前路径
print(os.getcwd()) #返回当前路径
with open("三国演义.txt", "r", encoding="utf-8") as source_file:
    with open("三国演义_bat.txt", "a", encoding="utf-8") as dst_file:#只读模式, 打开要复制的文件#追加模式, 打开副本文件
        while True:
            content = source_file.read(1024)
            if len(content) == 0:
                break
            dst_file.write(content)
# 或
# with open("三国演义.txt",'rt',encoding='utf-8') as f1:
#     with open("三国演义_bat.txt",'at',encoding='utf-8') as f2:
#         while True:
#             content = f1.read(1024)
#             if len(content) == 0:
#                 break
#             f2.write(content)